[en] The pressure tubes of Canadian deuterium uranium (CANDU) reactors are monitored to evaluate their health. The main factors that affect their service life are changes in dimensions, shape, mechanical properties, and flaw tolerance. The authors have developed equipment and techniques to measure these properties and assess the health of the tubes. In this paper they describe the examination of the Zircaloy-2 pressure tubes of the Nuclear Power Demonstration (NPD) reactor. Their dimensional changes posed no problems. However, the combined effect of irradiation and the presence of radial hydrides caused the fracture toughness to decrease to such an extent that it would be difficult to assure leak-before-break in the future. Thus the reactor was closed down after 25 years of operation. (author)